An eddy current deceleration device (which will hereinafter be referred to simply as a “deceleration device”) employing permanent magnets (which will hereinafter be referred to simply as “magnets”) includes a brake member fixed to a rotary shaft of a vehicle. In the eddy current deceleration device, in a braking state, eddy currents are generated in the surface of the brake member facing the magnets by the effect of magnetic fields of the magnets. Thereby, on the brake member that is rotating together with the rotary shaft, braking torque in a direction opposite to the rotational direction acts, and the rotational speed of the rotary shaft gradually decreases. Deceleration devices are classified into a drum type and a disk type according to the configuration of the brake member where eddy currents are generated, and the magnet holder which holds the magnets and is paired with the brake member. Generally, deceleration devices of the drum type are often used. For example, Japanese Patent Application Publication 2004-48963 (Patent Literature 1) discloses a drum-type deceleration device.
FIG. 1 is a longitudinal sectional view of a common drum type deceleration device. FIG. 2 is a perspective view showing the arrangement of permanent magnets in a conventional drum type deceleration device. FIGS. 3 and 4 cross-sectional views showing the generation status of magnetic circuits in the conventional deceleration device. FIG. 3 shows a braking state, and FIG. 4 shows a non-braking state. A longitudinal section means a section along the rotary shaft. A cross section means a section perpendicular to the rotary shaft.
As shown in FIG. 1, the deceleration device includes a cylindrical brake drum 1, and a cylindrical magnet holding ring 2 disposed inside the brake drum 1. The brake drum 1 is equivalent to the brake member to be provided with braking torque, and is fixed to a rotary shaft 10 (for example, a propeller shaft, a drive shaft or the like) of a vehicle via a rotor support 6. Accordingly, the brake drum 1 rotates together with the rotary shaft 10. The arrow in FIG. 1 shows an example of the rotational direction of the brake drum 1. The brake drum 1 has a radiator fin 1a on the outer periphery. The radiator fin 1a functions to cool the brake drum 1 itself. In the drawings other than FIG. 1, the radiator fin 1a is omitted.
The magnet holding ring 2 is equivalent to the magnet holder which is paired with the brake drum 1 (brake member), and is rotatably supported by the rotary shaft 10 via a stator support 7. The stator support 7 is fixed to a non-rotative member (for example, a transmission cover) of the vehicle.
As shown in FIGS. 1 and 2, a plurality of permanent magnets 3 are fixed on the outer peripheral surface of the magnet holding ring 2. The magnets 3 face the inner peripheral surface of the brake drum 1 with a gap, and the magnets 3 are arrayed in a circumferential direction throughout the whole circumference of a circle around the rotary shaft 10. The magnets 3 are laid such that the magnetic poles (the north pole and the south pole) of each of the magnets 3 are arranged in a radial direction from the axis of the rotary shaft 10 and such that the magnetic pole arrangements of circumferentially adjacent ones of the magnets 3 are opposite to each other. The magnet holding ring 2 is made of a ferromagnetic material.
As shown in FIGS. 1, 3 and 4, a plurality of ferromagnetic plate-like switches 4 are disposed in the gap between the brake drum 1 and the magnets 3. The plate-like switches 4 are arrayed in the circumferential direction throughout the whole circumference around the rotary shaft 10. The placement angles of the switches 4 are the same as the placement angles of the magnets 3. Both sides of the respective switches 4 are held by a switch holding ring 5. The switch holding ring 5 is fixed to the stator support 7.
To the switch holding ring 5, a drive unit (not shown) such as an air cylinder, an electric actuator or the like, is connected. For switching to a braking state or a non-braking state, the magnet holding ring 2 and the magnets 3 are rotated together by operation of the drive unit. In this way, the deceleration device can be switched between a braking state where each of the switches 4 entirely overlaps the magnet 3 immediately below (see FIG. 3) and a non-braking state where each of the switches 4 lies across two adjacent magnets 3 (see FIG. 4). Thus, the conventional deceleration device shown in FIGS. 2 to 4 employs, as a switching mechanism for switching between a braking state and a non-braking state, a structure in which the magnet holding ring 2 is rotatable around the rotary shaft 10. A switching mechanism having such a structure will hereinafter be referred to as a “single-row rotation switching mechanism”.
In the non-braking state, the single-row rotation switching mechanism operates to keep each of the switches 4 across two adjacent magnets 3 as shown in FIG. 4. In this state, the magnetic fluxes from the magnets 3 (magnetic fields of the magnets 3) are as follows. With regard to a first magnet 3 and a second magnet 3 that are adjacent to each other, the magnetic flux outgoing from the north pole of the first magnet 3 reaches the south pole of the second magnet 3 through the switch 4 therebetween. The magnetic flux outgoing from the north pole of the second magnet 3 reaches the south pole of the first magnet 3 via the magnet holding ring 2. Thus, no magnetic circuits are generated between the magnets 3 and the brake drum 1. In this state, no braking torque acts on the brake drum 1.
For switching to the braking state, the single-row rotation switching mechanism operates to rotate the magnet holding ring 2 by an angle that is about a half of the placement angle between two adjacent magnets 3. Thereby, each of the switches 4 is positioned to entirely overlap the magnet 3 immediately below as shown in FIG. 3. In this state, the magnetic fluxes from the magnets 3 (magnetic fields of the magnets 3) are as follows.
With regard to a first magnet 3 and a second magnet 3 that are adjacent to each other, the magnetic flux outgoing from the north pole of the first magnet 3 passes through the switch 4 located over the first magnet 3 and reaches the brake drum 1. The magnetic flux that has reached the brake drum 1 reaches the south pole of the second magnet 3 through the switch 4 located over the second magnet 3. The magnetic flux outgoing from the north pole of the second magnet 3 reaches the south pole of the first magnet 3 via the magnet holding ring 2. Thus, the circumferentially adjacent magnets 3 form a magnetic circuit across the adjacent magnets 3, the magnet holding ring 2, the switches 4 and the brake drum 1. Such magnetic circuits are formed throughout the whole circumference such that the directions of adjacent magnetic fluxes are opposite to each other. Then, on the brake drum 1 that is rotating together with the rotary shaft 10, braking torque in a direction opposite to the rotational direction acts.